A cutaway view of Jupiter, showing the expected metallic hydrogen “shell” {in Pink), held to be a key factor in its magnetic field [HT: Wikipedia]

A year ago, there was a happy announcement (and an article in Science), but doubts have since been entertained; indeed Science issued a correction to the original paper. Science:

>>Producing metallic hydrogen has been a great challenge in condensed matter physics. Metallic hydrogen may be a room-temperature superconductor and metastable when the pressure is released and could have an important impact on energy and rocketry. We have studied solid molecular hydrogen under pressure at low temperatures. At a pressure of 495 gigapascals, hydrogen becomes metallic, with reflectivity as high as 0.91. We fit the reflectance using a Drude free-electron model to determine the plasma frequency of 32.5 ± 2.1 electron volts at a temperature of 5.5 kelvin, with a corresponding electron carrier density of 7.7 ± 1.1 × 1023 particles per cubic centimeter, which is consistent with theoretical estimates of the atomic density. The properties are those of an atomic metal. We have produced the Wigner-Huntington dissociative transition to atomic metallic hydrogen in the laboratory.>>

The key point in the erratum, after discussing optical properties of diamonds (used as the “hammer and anvil” to squeeze the H) is: “corrected reflectance data in Fig. 3 and Table 1 must include multiple reflectance of light between the sample and the diamond table (8); there was a computational error in this part of the original analysis . . . updated data show that at temperature (T) = 5 K, the plasma frequency is 33.2 ± 3.5 eV (compared with the original 32.5 ± 2.1), which yields a carrier density of 81 ± 17 × 1022 electrons/cm3 (compared with the original 77 ± 11 × 1022 electrons/cm3.”

That’s a good example of the provisional nature of scientific findings, and of how a technical correction often looks. Worth noting for its own sake. (There actually are comments out there to the effect, the standards of peer review at Science need to be upgraded — such an article should never have been published, etc.)

But why the fuss and focus?

For one, there are suggestions of very interesting possible properties, especially if metallic hydrogen turns out to be metastable and superconducting at “normal” temperatures and pressures.

Wikipedia has a useful summary:

>>Though often placed at the top of the alkali metal column in the periodic table, hydrogen does not, under ordinary conditions, exhibit the properties of an alkali metal. Instead, it forms diatomic H2 molecules, analogous to halogens and non-metals in the second row of the periodic table, such as nitrogen and oxygen. Diatomic hydrogen is a gas that, at atmospheric pressure, liquefies and solidifies only at very low temperature (20 degrees and 14 degrees above absolute zero, respectively). Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 25 GPa (250000 atm; 3600000 psi) hydrogen would display metallic properties: instead of discrete H2 molecules (which consist of two electrons bound between two protons), a bulk phase would form with a solid lattice of protons and the electrons delocalized throughout.[2] Since then, producing metallic hydrogen in the laboratory has been described as “…the holy grail of high-pressure physics.”[10]

The initial prediction about the amount of pressure needed was eventually shown to be too low.[11] Since the first work by Wigner and Huntington, the more modern theoretical calculations were pointing toward higher but nonetheless potentially accessible metallization pressures of 100 GPa and higher.

Liquid metallic hydrogen

Helium-4 is a liquid at normal pressure near absolute zero, a consequence of its high zero-point energy (ZPE). The ZPE of protons in a dense state is also high, and a decline in the ordering energy (relative to the ZPE) is expected at high pressures. Arguments have been advanced by Neil Ashcroft and others that there is a melting point maximum in compressed hydrogen, but also that there might be a range of densities, at pressures around 400 GPa (3,900,000 atm), where hydrogen would be a liquid metal, even at low temperatures.[12][13]

In 1968, Neil Ashcroft suggested that metallic hydrogen might be a superconductor, up to room temperature (290 K or 17 °C), far higher than any other known candidate material. This hypothesis is based on an expected strong coupling between conduction electrons and lattice vibrations.[14]

Possibility of novel types of quantum fluid

Presently known “super” states of matter are superconductors, superfluid liquids and gases, and supersolids. Egor Babaev predicted that if hydrogen and deuterium have liquid metallic states, they might have quantum ordered states that cannot be classified as superconducting or superfluid in the usual sense. Instead, they might represent two possible novel types of quantum fluids: superconducting superfluids and metallic superfluids. Such fluids were predicted to have highly unusual reactions to external magnetic fields and rotations, which might provide a means for experimental verification of Babaev’s predictions. It has also been suggested that, under the influence of magnetic field, hydrogen might exhibit phase transitions from superconductivity to superfluidity and vice versa.[15][16][17]

Lithium alloying reduces requisite pressure

In 2009, Zurek et al. predicted that the alloy LiH6 would be a stable metal at only one quarter of the pressure required to metallize hydrogen, and that similar effects should hold for alloys of type LiHn and possibly other related alloys of type Lin.[18]>>

>>”One prediction that’s very important is metallic hydrogen is predicted to be meta-stable,” Silvera said. “That means if you take the pressure off, it will stay metallic, similar to the way diamonds form from graphite under intense heat and pressure, but remains a diamond when that pressure and heat is removed.”

Understanding whether the material is stable is important, Silvera said, because predictions suggest metallic hydrogen could act as a superconductor at room temperatures.

“That would be revolutionary,” he said. “As much as 15 percent of energy is lost to dissipation during transmission, so if you could make wires from this material and use them in the electrical grid, it could change that story.”

Among the holy grails of physics, a room temperature superconductor, Dias said, could radically change our transportation system, making magnetic levitation of high-speed trains possible, as well as making electric cars more efficient and improving the performance of many electronic devices.

The material could also provide major improvements in energy production and storage.>>

But, this case provides an example on the range of possibilities suggested by theoretical analysis and the cross-check from the messy and difficult realities of empirical investigation. For instance, it seems the diamond press broke under the pressures used, and the sample is no more. Epistemology counts in science, and empirical tests are often hard and expensive, but crucial. END